The present invention relates to a head of a wood club and more particularly to a golf club head which reduces the loss of flying distance even at the time of off-center hitting or when a ball is hit off the center of the striking face of the golf club.
In heads of most of wood clubs marketed recently, at least a face portion is made of a metallic material, and the thickness of the face portion has had to be thicker in order to maintain a strength which is good enough to resist an impact produced when a ball is hit thereby. Heads have continued to be made larger in size, resulting in a regulation that the volume of a head must not be larger than 460 cm3 with a permissible production error of 10 cm3, and hence, heads of most of the current drivers are enlarged as close to the upper limit of 460 cm3 as possible. When the head size is increased, the sweet spot is also enlarged, and the peripheral weight distribution is emphasized, whereby horizontal and vertical inertia moments are increased, and the occurrence of a hitting error such as an off-center hitting can be reduced. However, when the head size is increased to increase, in turn, the weight of the head, the head speed is reduced, and this sometimes results in a reduction in flying distance of a ball hit. To cope with this, a means has now been adopted to realize a head construction in which titanium (or an alloy thereof, hereinafter, both being referred to as “titanium”) which has a small specific weight and large strength, is used to form the whole of a head or a composite head is made of carbon and titanium.
In addition, not only golf clubs in which the head size is enlarged but also golf clubs with highly restitutive heads in which the restitution coefficient of the head is increased have been developed. As to the highly restitutive heads, the rule now stipulates that golf clubs with heads having a restitution coefficient of 0.830 or more are not allowed to be used in official tournaments from 2008 onward. Up until now, thick face materials have been aggressively used to increase the restitution coefficient in parallel with the adoption of those enlarged heads. However, even with those highly restitutive heads, when a ball is hit at a face portion other than the sweet spot or is hit at an off-center face portion, no spring effect can be expected, and the flying distance tends to be reduced drastically.
As a conventional example in which the spring effect is aimed by imparting springing properties to the whole of a face, there has been known a technique in which the thickness of a central portion including a sweet spot of a portion which makes up a face is formed to have a sufficient strength which can resist impact produced when the portion in question hits a ball, and the thickness of the periphery of the central portion is formed thinner than the central portion to impart springing properties to the whole of the face (refer to JP-A-9-192273 (page 2, FIG. 1).
With the golf club head described in JP-A-9-192273, although the flying distance of a ball is increased due to the springing properties of the whole of the face functioning properly when the ball is hit at the sweet spot, the flying distance of the ball is reduced remarkably when the ball is hit at the off-center portion on the face.